Quick reminder as Spring 21 goes live - if your http://Force.com  site breaks, don't panic.
First check the audit trail - that will tell you what changed, so you know where to start - in particular, what profile permissions/permissions set were changed/removed
1/x

Second - remember, a permission set will get removed if it contains any edit permission, even that isn't used on your site, but was added in error. So you might be able to remove the edit permission, and add the perm set back to the site, and off you go
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Third - if just you aren't seeing any records - check the sharing rules. The site might have been working because the Guest Profile had View All, so was ignoring your lack of sharing rules. Now you need to add them via the Sharing Rules section
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Fourth - if you need a page to have edit access, you'll need to make some code changes. Start here for some examples:

https://www.learnexperiencecloud.com/s/article/Guest-User-Record-Access-Development-Best-Practices
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You'll see several examples where they just change a class to Without Sharing and move on. That MIGHT work - but use with caution and make sure you understand the implications - without sharing can expose a lot of data without your meaning to
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If you are still stuck, join the Securing Experience Cloud group on success (even if you are just using regular http://force.com  sites!) - lots of conversations and solutions to read about and the best place to ask for help:
https://trailblazers.salesforce.com/_ui/core/chatter/groups/GroupProfilePage?g=0F93A0000004mDI
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All being well, this will be the last round of changes, and once you get it working again, we can all finally relax...and stay safe at home :)
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